Optomechanical mount for precisely steering/positioning a light beam

ABSTRACT

An optomechanical mounting includes an upper spring assembly and a lower spring assembly that support and secure a sphere containing an optical element. The materials in the mounting have the same or nearly the same CTEs and spring assemblies provide opposing radial forces so that thermal expansions are compensated, giving the mounting superior thermal stability. Frictional forces on the sphere from the upper and lower spring assemblies maintain the orientation of the sphere (and the optical element) during operation, but smooth surfaces of the sphere and springs still permit sensitive, precision rotation of sphere for alignment without post-alignment clamping of the sphere. The spring assemblies can be ring-shaped to permit an opening through the spring assembly to the sphere for light paths or for tools that adjust the alignment of the sphere.

CROSS REFERENCE TO RELATED APPLICATION

[0001] This application is a continuation-in-part of U.S. patentapplication Ser. No. 09/906869 of Kenneth J. Wayne et al., filed Jul.16, 2001, entitled “Optomechanical Mount for PreciselySteering/Positioning a Light Beam.”

BACKGROUND

[0002] Many optical systems require precision optomechanical mountingsthat hold optical elements in the positions and orientations requiredfor operation of the system. To achieve proper positioning and alignmentof an optical element, an optomechanical mounting generally must allowmovement or rotation of the optical element relative to other opticalelements during an alignment process, but once the optical element isaligned the mounting must securely hold the optical element to maintainthe proper alignment during shipping and use of the optical system. Aconflict exists between the need to align an optical element with greatprecision and the need to have the optical element remain aligned forthe lifetime of the optical system.

[0003] To provide adjustability for alignment and still securely holdthe optical element in position, optomechanical mounts often useclamping systems. In particular, an optical element in theoptomechanical mounting can be adjusted or aligned when the clamp isloose, but the optical element is rigidly held when the clamp istightened. One concern in optomechanical mounts using clamps isdisturbance of the optical element's alignment when clamping theoptomechanical mount.

[0004] Using the same clamping friction during alignment and use of anoptical system avoids disturbances that arise from post-alignmentclamping but requires a tradeoff between alignment precision andoperational stability. In particular, a multi-axis interferometertypically requires optomechanical mounts that can precisely orient laserbeams with sub-microradian sensitivity. An optomechanical mount with apure kinematic design can provide sub-microradian sensitivity, buttypically cannot hold that alignment when subjected to shock,vibrations, and temperature changes. A ruggedly clamped, and thereforestable, optomechanical mount generally is difficult to adjust withsub-microradian precision. Accordingly, tradeoffs are generally requiredbetween the precision of adjustments and the stability, and asemikinematic design is often the compromise.

[0005]FIG. 1 shows an optomechanical mounting 100 that emphasizesminimal constraint and adjustment sensitivity. Such optomechanicalmountings are further described in U.S. Pat. No. 6,170,795.Optomechanical mounting 100 includes a support 14, a three-sphere nest20, a sphere 12, a top plate 14, and a spring preloaded plunger 26(spring not shown) operated by a clamp screw 28. Sphere 12 contains anoptical element such as a mirror (not shown) that can be rotated onthree-sphere nest 20 for an alignment process that changes theorientation of the optical element without changing the position of itsoptical center.

[0006] Alignment of the optical element, which is fixed at the center ofsphere 12, generally requires loosening clamp screw 28 to relieve orreduce the clamping force on sphere 12 and permit rotation of sphere 12.The spring (not shown) bearing on plunger 26 thus applies an initialstabilizing force directed along a radius of sphere 12. Once sphere 12is aligned, tightening clamp screw 28 overcomes the spring and causesplunger 26 to apply the clamping force, which holds sphere 12 in theproper orientation. The final clamping force is set by applying aprescribed torque to clamp screw 28, and the frictional forces resultingfrom the clamping force resist rotation of sphere 12 to retain thealignment of the optical element.

[0007] The need to loosen clamp screw 28 before alignment and the needto tighten clamp screw 28 after alignment increase the total timerequired for the alignment process. Additionally, clamp screw 28 may notbe easily accessible in an optical system, which makes the alignmentprocess more difficult. Additionally, tightening clamp screw 28 causesbending of the assembly and hence disturbs the accuracy of thejust-completed alignment. For most applications, an optomechanicalmounting is desired that maintains precise angular orientation of theoptical element without requiring additional procedures to apply aclamping force after the alignment process.

SUMMARY

[0008] In accordance with an aspect of the invention, an optomechanicalmounting includes an upper spring assembly and a lower spring assemblythat support and secure a sphere containing an optical element fixed atits center. The spring assemblies can be substantially identical so thatthermal expansions affecting one assembly compensates for identicalopposing thermal expansion affecting the other spring assembly, givingthe optomechanical mounting superior thermal stability. Frictionalforces on the sphere from the upper and lower spring assemblies maintainthe orientation of the sphere (and the optical element in the sphere)during operation but still permit rotation of the sphere for alignmentwithout removing either spring assembly or releasing the spring tensionthat the spring assemblies apply to the sphere.

[0009] Each spring assembly can include springs around a perimeter of aring so that a central region of each assembly is open for an opticalpath through the assembly. Alternatively, the central region can be opento provide access for tools that facilitate rotation the sphere foralignment of the optical element.

[0010] Embodiments of the invention can exhibit excellent long-termalignment stability when subjected to temperature changes, shock, andvibration. The symmetry of the optomechanical mount and the use ofsimilar construction materials in the elements of the mount provideexcellent thermal stability. High clamping forces between the springsand the sphere resist alignment changes caused by mechanical shock. Inparticular, frictional forces at multiple points on the sphere resistrotation of the sphere after alignment is achieved, but fine surfacefinishes on the sphere and spring make smooth, high resolutionrotational adjustment achievable with removable alignment tools.Vibration stability results because the springs, which stiffen due togeometrical deformation as a result of high compressive forces, wraptightly around the sphere to provide a stiff, highly damped spring/masssystem having a high resonant frequency, typically greater than 3 kHz.

[0011] One specific embodiment of the invention is a system thatincludes a sphere adapted for mounting an optical element, a first setof springs including multiple springs in contact with the sphere; and asecond set of springs including multiple springs in contact with thesphere. Generally, each spring in the first set has a correspondingspring in the second set, and each spring in the first set applies aforce to the sphere that is collinear with and opposite to a force thatthe corresponding spring in the second set applies to the sphere. Allspring forces are directed through the center of the sphere. Theopposing forces from the springs maintain positional stability of thesphere when the optomechanical mounting is subjected to thermalvariations, vibrations, or shock.

[0012] Typically, either set of springs can be mounted on an innersurface of a support ring. The inner surface of the support ring istypically a conic section with fixtures for mounting the springs, andeach spring can be a leaf spring set at an angle according to the normalto the sphere's surface where the spring contacts the sphere. Thesupport rings can be open in the center for light paths of the opticalelement or for access to the sphere during the alignment process. A caseon which the support rings are mounted can control the separation of thefirst and second set of springs or their associated support rings tocontrol magnitudes of forces that the first and second sets of springsapply to the sphere.

[0013] Another embodiment of the invention is a system wherein springwashers are used to support and apply force on the sphere. Fewer springsare required, and no fixtures need to be created on the inner surface ofthe support ring.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1 is a cross-sectional view of a known optomechanicalmounting.

[0015]FIG. 2 is a perspective view of a lower spring assembly for anoptomechanical mounting in accordance with an embodiment of theinvention.

[0016]FIG. 3 is a perspective view of a sphere containing an opticalelement and positioned on the lower spring assembly of FIG. 2.

[0017]FIG. 4 is a perspective view of an upper spring assembly, a spherecontaining an optical element, a lower spring assembly, and a base foran optomechanical mounting in accordance with an embodiment of theinvention.

[0018]FIG. 5 is a perspective view of a complete optomechanical mountingin accordance with an embodiment of the invention.

[0019]FIGS. 6A and 6B are perspective views showing the relativeorientations of supporting springs in alternative embodiments of theinvention.

[0020]FIG. 7 shows a sphere for containing a refractive translator opticfor a mounting in accordance with an embodiment of the invention.

[0021]FIGS. 8A and 8B show a sphere for containing a reflective beambender/splitter for a mounting in accordance with another embodiment ofthe invention.

[0022]FIGS. 9A and 9B illustrate a sphere positioned between two springwashers in accordance with another embodiment of the invention. FIG. 9Bis a cross-sectional view of the objects in FIG. 9A, taken along avertical plane passing through the line B-B′ shown in FIG. 9A. Supportrings have been added to this view, and the sphere cross-section isrepresented by a circle to keep the picture clear and easily understood.

[0023] Use of the same reference symbols in different figures indicatessimilar or identical items.

DETAILED DESCRIPTION

[0024] In accordance with an aspect of the invention, upper and lowerspring assemblies support a sphere containing an optical element. Thespring assemblies are substantially identical and are oriented so thateach spring applies a force along a radius of the sphere. Each of thesesforces is collinear with an opposing force from a spring in the otherspring assembly. The springs accordingly hold the sphere in positionwith a high degree of thermal stability because thermal expansion thatchanges a force from one spring assembly is matched by a thermal changein the opposing force from the other assembly.

[0025] Each spring assembly can include springs on a support ring with acentral portion of the assembly opened for an optical path or alignmenttool access.

[0026]FIG. 2 is a perspective view of a lower spring assembly 200 inaccordance with an exemplary embodiment of the invention. Springassembly 200 includes three leaf springs 220 attached to a support ring210.

[0027] Support ring 210 is made of a rigid material such as tool steeland is predominantly circular with a conical inner surface. In theexemplary embodiment, support ring is about 14 mm thick and has an outerdiameter of about 44.22 mm. The inner surface has a cylindrical portionwith a diameter of about 25.7 mm and height of about 6.82 mm at thebottom of support ring 210. A conical portion extends upward at a 45°from the cylindrical portion. Accordingly, the conical portion has anopening at the top support ring 210 of about 39.92 mm.

[0028] Located 120° at-intervals around the inner surface of supportring 210 are fixtures 230 for seating and attaching leaf springs 220. Inthe exemplary embodiment of the invention, each leaf spring 220 is arectangular piece of flat metal such as spring steel about 20.70 mm wideand 12.7 mm high with a thickness of about 0.00762 mm. Leaf springs 220are flat in the exemplary embodiment of the invention but can be convexor concave in alternative embodiments of the invention.

[0029] Each fixture 230 is machined into the conical portion of theinner surface of support ring 210 and sized to accommodate a leaf spring220. In the exemplary embodiment, fixtures 230 provide flat ledges about4.15 mm wide, and leaf springs 220 are welded to ledges 232 by stitchwelds 234 that are no more than 4.0 mm long to avoid welds extendingbeyond ledges 232. A locating pin 240, which is not a part of springassembly 200, can help position springs 220 for attachment (e.g.,welding) to support ring 210, and in the exemplary embodiment, locatingpin 240 has a diameter of about 20.08 mm. In other embodiments, othermeans such as epoxy or spring tension can hold leaf springs 220 tosupport ring 210, or leaf springs can be free floating in the fixtures230 of support ring 210.

[0030]FIG. 3 shows a sphere 300 resting on lower spring assembly 200.Sphere 300 contains an optical element (not shown) such as a mirror, abeam splitter, a translating window, a wedge window, or a lens. In theexemplary embodiment, sphere 300 is a precision bearing about 41.275 mmin diameter that is machined to include an opening 310 for the opticalelement, openings 320 for light paths to and from the optical element.Sphere 300 can further include openings 330 that fit an alignment toolsuch as an Allen key or lever that can be used to rotate sphere 300 inthe finished optomechanical mounting.

[0031] To assemble the optomechanical mounting, lower assembly 200 isattached (e.g., bolted) to a base plate 410 as illustrated in FIG. 4.Mounting feet of base plate 410 can be designed to flex rather than slipwhen the mount encounters differential thermal expansion, e.g., from athermal gradient or when base plate 410 is attached to a base of adifferent material. Thus, when the temperature returns to normal theoriginal alignment of the optical element is reestablished.

[0032] Sphere 300 is placed on lower spring assembly 200. An upperspring assembly 420, which sits on sphere 300 is attached inside a cover510 shown in FIG. 5, and cover 510 is attached to base plate 410 so thatupper spring assembly 420 contacts sphere 300 as shown in FIG. 4.

[0033] The height of cover 510 of FIG. 5 and the gap between cover 510and base plate 410 are selected so that cover 510 and base plate 410apply pressure to upper and lower spring assemblies 420 and 200. In theexemplary embodiment of the invention, cover 510 has a cavity of height42.83 mm that contains lower spring assembly 200, sphere 300, and upperspring assembly 420. As a result, in optomechanical mounting 500, thesprings in upper spring assembly 420 and lower spring assembly 200contact and apply radial forces to sphere 300.

[0034] All of the components in optomechanical mounting 500 can be madeof the same material or materials that are substantially the same or atleast have the same or similar coefficients of thermal expansion (CTEs).If the CTEs are the same, the entire assembly expands or contracts inunison when subjected to a temperature change. Thus, sphere 300 will notrotate during acclimatization, and the angular alignment of the opticalelement is preserved when the temperature changes.

[0035] In the exemplary embodiment, upper spring assembly 420 issubstantially identical to lower spring assembly 200, but theattachments of lower spring assembly 200 to base plate 410 and upperspring assembly 420 to cover 500 orient springs 620-1, 620-2, and 620-3(shown in FIG. 6A ) of upper spring assembly 420 directly oppositecorresponding springs 220-1, 220-2, and 220-3 (shown in FIG. 2) alongrespective lines through the center of sphere 300 as shown in FIG. 6A.For example, springs 220-1, 220-2, and 220-3 can be located at 0°, 240°,and 120° around a vertical axis of sphere 300, while springs 620-1,620-2, and 620-3 are located at 180°, 60°, and 300° around the verticalaxis. With this configuration, spring force vectors for a pair ofsprings (220-1, 620-1), (220-2, 620-2), or (220-3, 620-3) are collinearand pass through the center of sphere 300. Each spring 220 or 620 thushas a corresponding spring 620 or 220 that provides an equal, collinearopposing force through the center of sphere 300. Accordingly, springs220 and 620 do not apply a torque to sphere 300, and changes in springs620, for example, caused by changes in temperature, counter or cancelcorresponding changes in springs 220 to keep sphere 300 from shiftingposition.

[0036]FIG. 6B illustrates the spring configuration in an alternativeembodiment of the invention. The embodiment of FIG. 6B uses four springs631, 632, 641, and 642 Two springs 631 and 632 are in a lower springassembly (not shown), and two springs 641 and 642 in an upper springassembly (not shown). Each spring 631, 632, 641, and 642 is a leafspring having a surface perpendicular to the normal to the surface ofsphere 300 at the respective contact points. The upper and lower springassemblies are identical to each other, but the upper spring assembly isrotated by 90° relative to the lower spring assembly so that the contactpoints of sphere 300 with springs 631, 632, 641, and 642 are at thevertices of a symmetric tetrahedron. The resulting spring forces onsphere 300 are directed radially toward the center of sphere 300 andhold sphere 300 without applying a torque to sphere 300. Other springconfigurations could similarly hold sphere 300 without applying atorque, for example, a single spring on top of sphere 300 could opposeand counter the resultant of the three forces from springs 220. However,such configurations lack the symmetry of the system of FIG. 6A, whereeach spring is paired with an opposing spring. Accordingly, thermalexpansion of a mount using a minimally constrained arrangement such asillustrated in FIG. 6B may change the position or orientation of sphere300.

[0037] Returning to FIG. 5, cover 510 has openings 520 for light pathsto and from the optical element in sphere 300 and openings 530 for toolsused to rotate sphere 300 during an alignment process. Additionally,since spring assembly 420 is ring-shaped, an opening 540 in the top ofcover 500 or bottom plate 410 can allow a light path through the top ofcover 510 or allow access to the top of sphere 300 for a tool used torotate sphere 300 during an alignment process. Similarly, access can beprovided through base plate 410 to the bottom of sphere 300.

[0038] Optical elements mounted in a sphere 300 can vary widely, butgenerally, the center of sphere 300 lies on an optical surface, an axis,and/or a symmetry plane of the optical element in sphere 300. FIG. 7,for example, illustrates a sphere 700 containing a refractive translatoroptic 710 for shifting the position of a laser beam perpendicular todirection of propagation of the laser beam. Translator 710 can simply bea thick piece of optical quality glass having parallel optical surfaces.An Allen key fit into an opening 720 can be used to rotate sphere 700 inan optomechanical mounting such as described above to change theincidence angle of an input beam and control the amount of shiftrefraction causes in translator 700. Additional access ports for toolingcan be provided at almost any position, notably at 45° positions in avertical plane.

[0039]FIGS. 8A and 8B show perspective views of a sphere 800 used for abeam bender. Sphere 800 has an opening into which an optical element 810having a highly reflective surface 812 is inserted and attached.Openings 820 are for an input beam and a reflected beam that enter andexit sphere 800. Openings 830, which are accessible through openings inthe cover of the optomechanical mounting, allow a lever to rotate sphere800 as required to align highly reflective surface 812 with the inputbeam. Optical element 810 is positioned so that highly reflectivecoating 812 is at the center of sphere 800 so that rotation of sphere800 changes the angle of incidence of the input beam on surface 812 butdoes not change the point of incidence on surface 812.

[0040]FIG. 9A illustrates another embodiment of the invention usingspring washers 900. The sphere 300 is positioned on a lower springwasher 910, such that the lower spring washer 910 contacts the sphere300 along a longitudinal circle on the surface of the sphere 300. Anupper spring washer 920 is placed opposite lower spring washer 910, suchthat the upper spring washer 920 also contacts the surface of the sphere300 along a longitudinal circle.

[0041] The spring washers 900 are preferably made out of metal such asspring steel, stainless steel, or carbon steel. For some applications,the spring washers 900 should be polished to lower surface frictionduring alignment of the sphere 300. The spring washers 900 shown in theexemplary embodiment are Belleville spring washers, a type of washerwell known in the art. Other washers, such as modified Belleville springwashers, can also be used. The spring washers 900 in the exemplaryembodiment have an outer diameter of about 36 mm, an inner diameter ofabout 22.4 mm, and a thickness between 0.1 mm and 0.3 mm.

[0042]FIG. 9B is a cross-sectional view of the sphere 300, springwashers 900, and support rings 940 (collectively known as assembly 930)as assembled in accordance with this embodiment of the presentinvention. Lower support ring 950 and upper support ring 960 are similarto the support ring 210 shown in FIG. 2; each of the support rings 940is predominantly circular, but the inner surface is cylindrical ratherthan conical, and does not have fixtures 230 for holding leaf springs220 that are found in support ring 210. The spring washers 900 arepositioned on the edge of the cylindrical inner surface of the supportrings 940. In an alternate embodiment, the inner surface of the supportrings 940 is slightly conical to help keep the spring washers 900 inplace. The spring washers 900 are fastened in well-known ways (e.g.,welded) to the support rings 940. Alternatively, the spring washers 900can be left unfastened to the support rings 940.

[0043] To assemble the optomechanical mounting, the assembly 930 isattached to a base plate 410 (shown in FIG. 5). A cover 510 (shown inFIG. 5) is placed over assembly 930, such that cover 510 and base plate410 apply force on support rings 940. The gap between cover 510 and baseplate 410 can be varied to vary the magnitude of force applied. Thesupport rings 940 apply force on the spring washers 900, such that theupper spring washer 920 exerts an equal, collinear opposing force onsphere 300 against the lower spring washer 910.

[0044] Although the invention has been described with reference toparticular embodiments, the described embodiments are only examples ofthe invention's application and should not be taken as limitations. Forexample, although specific dimensions and materials were described foran exemplary embodiment of the invention, those dimensions and materialsare subject to wide variations and replacements. Various otheradaptations and combinations of features of the embodiments disclosedare within the scope of the invention as defined by the followingclaims.

We claim:
 1. An optomechanical system comprising: a sphere adapted formounting an optical element in the sphere; a first set of one or moresprings in contact with the sphere; and a second set of one or moresprings in contact with the sphere.
 2. The system of claim 1, whereineach spring in the first set comprises a spring washer.
 3. The system ofclaim 1, wherein each spring in the first and second sets comprises aspring washer.